There is an ongoing interest in the development of absorbent articles such as paper towels. Disposable paper towels are widely used in the home for wiping spills, especially of water or watery liquids; for cleaning work-surfaces such as those of the kitchen and bathroom; for food preparation and handling; or for cleaning glass. More generally, absorbent papers are sometimes incorporated into other absorbent articles, such as dressings, catamenials and disposable diapers.
Manufacturing, more specifically sheet-forming, processes for paper are well-established in commerce. Papermaking machinery is very capital-intensive, and as a result, improvements in absorbent paper which do not require any major change in, or complication of, the paper-forming process tend to be highly appreciated. The processes of major importance include air-laying and wet-laying.
In outline, the latter process involves filtering a dilute dispersion of fibers onto a mesh (usually termed a Fourdrinier wire) and drying the resulting web. There is a large installed base of manufacturing equipment using continuous, high-speed machinery based on the wet-laying technique, representing considerable investment.
Conventional papermaking fibers suitable for wet-laying papermaking are cellulosic fibers which disperse well and can readily be filtered and dried. They typically absorb relatively small amounts of water, of the order of a few grams per gram of bone dry fiber.
The simplest notion for improving the absorbency of paper involves adding thereto a highly water-absorbent material, such as one of the gel-forming polycarboxylate polymers, which are well-known in the art. Very high absorbencies are possible, of the order of hundreds of grams of water per gram of polymer. Such materials have found particular utility as disposable diaper "superabsorbents".
Superabsorbents are however inherently very difficult to handle in a wet-laying process. Thus they tend to disintegrate under the relatively high shear forces involved in wet-laying papermaking. Moreover, they are difficult to filter, tending to block the Fourdrinier wire; and once deposited, they are very difficult to dry. The final product tends to be stiff and may not rewet to an acceptable degree when in use.
Applying the above-identified simple absorbency-improving notion to processes other than wet-laying has led to the development of a laminated structure or "sandwich" having outer plies of conventional paper and an inner layer consisting essentially of superabsorbent. However, the superabsorbent tends to leak out from the product paper structure, especially through pinholes or when the paper structure is torn. Slippery, gel-like material is released, which is a serious aesthetic disadvantage.
The deficiencies of the above approaches suggest a need to consider more than just the absolute magnitude of the absorbency which can be had from a particular paper additive. Thus, although absorbency is of primary importance, other requirements, such as ease of manufacture and product aesthetics, must be met. In addition, as distinct from water-absorbing capacity, another problem which has been identified in the context of absorbent structures has to do with rate of water uptake, in technical terms "wicking rate". Wicking rate is particularly important in a disposable paper towel which must quickly absorb a spill.
In principle, it is possible to suggest making absorbent paper by wet-laying an improved absorbent fiber (as distinct from particulate superabsorbents on one hand or conventional papermaking fibers on the other). Indeed, there are numerous literature reports of "absorbent fibers". However it would seem that these often involve mere physical coating of a fiber, such as processes involving precipitating absorbent polymers onto fibers or polymerizing monomers such as acrylic acid and methylenebisacrylamide in the presence of a fiber. In such situations, the chemical means are not present to covalently attach the polymer to the fiber. In consequence, the coating may not survive the shear forces involved in typical wet-laying operations, and may come off, the result being wire-blocking and/or drying problems similar to those mentioned above.
Grafted fibers are also well-known. Typical of grafted fibers are those made by graft copolymerizing methyl acrylate and cellulosic fibers in the presence of an appropriate catalyst such as cerium(IV) ammonium nitrate followed by hydrolyzing to the absorbent form. Absorbent grafted fibers are often not as strong as might be desired for wet-laying papermaking, since the low molecular weight monomer used in the preparation is capable of penetrating the fiber, polymerizing in the interior, so that when hydrolyzed and exposed to water, the fiber "balloons" internally and can easily shatter.
Various highly absorbent polymers have been extruded, and the extrudates have sometimes been termed "fibers". However, these materials are in fact not fibers in the usual sense of cellulosic papermaking fibers, rather they tend to be chemically homogeneous, and as with the common particulate superabsorbents, form slippery gels and encounter processing problems when wet-laid.
Oddly, to add to the above, there are reports in the literature of various chemicals apparently similar to those used herein apparently imparting wet-strength and/or hydrophobicity, i.e., water-resistance, to paper.
In view of the foregoing considerations, improvements in absorbent cellulosic fibers which do not make the fiber incompatible with wet-laying are highly desirable.
Accordingly, it is an object of the instant invention to provide a wet-layable papermaking fiber and pulp having an improved absorbent form.
More specifically, it is an object herein to provide a chemically modified fiber (hereinafter "the fiber of the invention") having three chemically bonded components, namely a cellulose of natural origin (such as an ordinary pulp fiber), a poly(methyl vinyl ether-co-maleate) copolymer and a polyol; which fiber has a water-absorbent chemical form (such as the sodium salt form), which is useful especially in that it is readily capable of being distributed into a web by wet-laying (e.g., as a pulp) in admixture with untreated fibers.
It is another object of the invention to provide absorbent wet-laid paper comprising the fiber of the invention.
A further object of the invention is the provision of a suitable process, unreliant on metal catalysts as used in common grafting processes of the art, for reproducibly making the fiber of the invention.
These and other objects are secured, as will be seen from the following disclosure.